Optical semiconductor device and method for manufacturing optical semiconductor device

ABSTRACT

An optical semiconductor device includes: a photo detector section which includes: a first semiconductor layer of a first conductivity type formed on a surface of a semiconductor substrate of the first conductivity type, a second semiconductor layer of a second conductivity type formed on a surface of the first semiconductor layer, and an antireflection film formed on a surface of the second semiconductor layer and preventing reflection of incident light; and a circuit element section which includes: a circuit element formed on the second semiconductor layer on the semiconductor substrate, and a passivation film covering an uppermost electrode layer among electrode layers constituting the circuit element and formed out of a same material as a material of the antireflection film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.10/320,684, filed Dec. 17, 2002 now U.S. Pat. No. 6,791,153, and basedupon and claims the benefit of priority from the prior Japanese PatentApplication No. 2002-63188, filed on Mar. 8, 2002. The contents of theseapplications are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical semiconductor device and amethod for manufacturing the optical semiconductor device.

2. Related Background Art

A photodiode of a PIN structure is conventionally employed as a photodetector which converts an optical signal used in optical communicationor a DVD and the like into an electrical signal.

The PIN-type photodiode has a structure in which a so-called i(intrinsic) layer consisting of a semiconductor having a relatively lowimpurity concentration is put between p and n semiconductors havingrelatively high impurity concentrations.

A bipolar transistor, a capacitor, a resistance, a MOSFET and the likeare used as signal-processing circuit elements which process anelectrical signal from the photo-detector.

An optical semiconductor device is conventionally formed by hybridizinga PIN photodiode and signal-processing circuit elements formed ondifferent semiconductor substrates or semiconductor chips, respectively(such optical semiconductor device will be referred to as“hybrid-optical semiconductor device” hereinafter).

Further, there is known an optical semiconductor device which has a PINphotodiode and a signal-processing circuit formed on the samesemiconductor substrate or semiconductor chip (such opticalsemiconductor device will be referred to as “single-substrate-typeoptical semiconductor device” hereinafter).

The single-substrate-type optical semiconductor device has fewer partsthan those of the hybrid-optical semiconductor device in an assemblyprocess and requires fewer steps in the assembly process. Therefore, thesingle-substrate-type optical semiconductor device can reducemanufacturing costs more than the hybrid-optical semiconductor device.Further, the single-substrate-type optical semiconductor device does notrequire a bonding wire that connects from a semiconductor chip on whicha PIN photodiode is formed to a semiconductor chip on which asignal-processing circuit is formed. Therefore, thesingle-substrate-type optical semiconductor device can resist externalelectromagnetic noise better than the hybrid-optical semiconductordevice. As a consequence, the single-substrate-type opticalsemiconductor device is more advantageous than the hybrid-opticalsemiconductor device.

FIG. 8 is a schematic enlarged cross-sectional view of a conventionalsingle-substrate-type optical semiconductor device. As shown therein, ap⁻-type epitaxial layer 12 is formed on a p-type semiconductor substrate10. An n-type epitaxial layer 16 is formed on the epitaxial layer 12. Aninsulating layer 18, an insulating layer 20, an electrode layer 22, apassivation film 24 and a passivation film 26 are sequentially providedon the epitaxial layer 16 in this order.

On the epitaxial layers 12 and 16, various diffused layers 14, 40, 42and 44 are provided to form a photodiode section 50 and asignal-processing circuit section 60. In addition, electrodes 28 and 29connected to the diffused layers through the insulating layer 18 areformed on the epitaxial layers 16.

The electrode layer 22 is a metal layer electrically connected to one ofthe electrodes formed on the epitaxial layer 16 and also functions as alight-shielding film which shields the signal-processing circuit sectionfrom light. Therefore, in the optical semiconductor device 200, theelectrode layer 22 is not formed in the photodiode section 50 and lightis allowed to be incident only on this photodiode section 50.

However, the insulating layers 18 and 20 and the passivation films 24and 26 used to manufacture the signal-processing circuit section 60, theelectrode 28 and the like are formed on the surface of the epitaxiallayer 16 in the photodiode section 50. Because of the presence of theinsulating layers 18 and 20 and the passivation films 24 and 26, most ofthe incident light incident on the photodiode section 50 is reflected.As a result, the quantity of light incident on portions below thesurface of epitaxial layer 16 is decreased. Due to this, the photosensitivity of the optical semiconductor device 200 disadvantageouslydeteriorates.

Furthermore, the film formed on the surface of the epitaxial layer 16 inthe photodiode section 50 is a multilayer film which consists of theinsulating films 18 and 20 and the passivation films 24 and 26 differentfrom one another in property and thickness. Since the respective filmsof this multilayer film are formed in different manufacturing steps fromone another, the material, property and film thickness vary among thesefilms. As a result, the reflectance of the incident light incident onthe photodiode section 50 is not kept constant. Due to this, thereoccurs the problem that the photo sensitivity of the opticalsemiconductor device 200 has a disadvantageously large variation.

As stated above, the reflectance for reflecting the incident lightincident on the photodiode section 50 is largely influenced by thematerials, properties and thicknesses of the films covering the surfaceof the epitaxial layer 16. However, it is difficult to form the filmshaving different materials, properties and thicknesses on the epitaxiallayer 16 so as to minimize reflectance in view of the refractive indexof the epitaxial layer (e.g., the refractive index of silicon≈3.44) andthe wavelength of the incident light.

In addition, Japanese Patent Application Publication No. 4-271173discloses an optical semiconductor device having a dielectric thin filmand an antireflection film which have common properties and thickness,and which are manufactured in a common manufacturing step. Thedielectric thin film is used between the electrodes of the capacitor ofa peripheral circuit element. The antireflection film is used in a photodetector.

In the optical semiconductor device disclosed in PublicationNo.HEI4-271173 (1992), however, the thickness of the antireflection filmis a factor that determines the capacitance of the capacitor. Therefore,the thickness of the antireflection film is limited by the capacitanceof the capacitor. If the thickness of the antireflection film is set atan optimum thickness in accordance with the wavelength of incidentlight, the areas of the electrodes of the capacitor have to be changedso as to obtain a desired capacitance.

Furthermore, in the optical semiconductor device disclosed inPublication No. 4-271173, the antireflection film of the photo detectoris formed when the dielectric thin film used between the electrodes ofthe capacitor is formed. Due to this, such films as passivation filmsare formed on the antireflection film of the photo detector. As aresult, there occurs the problem that in order to control thereflectance in the photo detector, it is disadvantageously necessary tocontrol not only the thickness of the antireflection film but also thatof the passivation films on the antireflection film.

Therefore, it is desired to provide an optical semiconductor devicewhich has a relatively high photo sensitivity and which can reduce thevariation of photo sensitivity even if a photo detector and a circuitelement are formed on the same semiconductor substrate, and to provide amethod for manufacturing the optical semiconductor device.

It is also desired to provide an optical semiconductor device which cancontrol a photo sensitivity relatively easily without influencing acircuit element even if a photo detector and a circuit element areformed on the same semiconductor substrate, and to provide a method formanufacturing the optical semiconductor device.

It is further desired to provide a method for manufacturing an opticalsemiconductor device which enables a photo detector and a circuitelement having relatively high photo sensitivity and small variation inphoto sensitivity to be manufactured on the same semiconductorsubstrate, and to provide the optical semiconductor device.

SUMMARY OF THE INVENTION

An optical semiconductor device according to an embodiment of thepresent invention, the optical semiconductor device comprises: a photodetector section including a first semiconductor layer of a firstconductivity type formed on a surface of a semiconductor substrate ofthe first conductivity type, a second semiconductor layer of a secondconductivity type formed on a surface of the first semiconductor layer,and an antireflection film formed on a surface of the secondsemiconductor layer and preventing reflection of incident light; and

a circuit element section including a circuit element formed on thesecond semiconductor layer on the semiconductor substrate, and apassivation film covering the circuit element and having a passivationfilm formed out of a same material as a material of the antireflectionfilm.

A method for manufacturing the optical semiconductor device according tothe embodiment of the present invention, is the method for manufacturingthe optical semiconductor device constituted so that a photo detectorsection which receives light and generates a photocurrent and a circuitelement section which processes a signal based on the photocurrent fromat least the photo detector section are formed on a same semiconductorsubstrate, the method comprising: a step of forming a firstsemiconductor layer of a first conductivity type on a surface of thesemiconductor substrate of the first conductivity type; a step offorming a second semiconductor layer of a second conductivity type on asurface of the first semiconductor layer; a diffused layer formationstep of selectively forming diffused layers in the second semiconductorlayer in the photo detector section and the circuit element section; aninsulating film formation step of depositing a first insulating film onthe second semiconductor layer; an exposure step of exposing the secondsemiconductor layer in a light-receiving region which receives the lightin the photo detector section; and a passivation film formation step offorming an antireflection film which prevents reflection of incidentlight on the second semiconductor layer in the light-receiving region,and forming a passivation film which is made of a same material as amaterial of the antireflection film and covers the circuit element abovethe first insulating film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic partially enlarged cross-sectional view of anoptical semiconductor device 100 in an embodiment according to thepresent invention;

FIG. 2 is a partially enlarged cross-sectional view showing amanufacturing step of a method for manufacturing the opticalsemiconductor device in the embodiment according to the presentinvention;

FIG. 3 is a cross-sectional view of the elements in a manufacturing stepfollowing the step shown in FIG. 2;

FIG. 4 is a cross-sectional view of the elements in a manufacturing stepfollowing the step shown in FIG. 3;

FIG. 5 is a cross-sectional view of the elements in a manufacturing stepfollowing the step shown in FIG. 4;

FIG. 6 is a cross-sectional view of the elements in a manufacturing stepfollowing the step shown in FIG. 5;

FIG. 7 is a cross-sectional view of the elements in a manufacturing stepfollowing the step shown in FIG. 6; and

FIG. 8 is a schematic enlarged cross-sectional view of a conventionalsingle-substrate-type optical semiconductor device 200.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment according to the present invention will be describedhereinafter with reference to the drawings. It is noted, however, thatthe embodiment is not intended to limit the present invention. Inaddition, in the embodiment described below, even if an n-typesemiconductor is employed in place of a p-type semiconductor and ap-type semiconductor is employed in place of an n-type semiconductor,the same advantages as those of the present invention or the embodimentcan be obtained.

FIG. 1 is a schematic partially enlarged cross-sectional view of anoptical semiconductor device 100 in an embodiment according to thepresent invention. The optical semiconductor device 100 includes ap-type semiconductor substrate 10, a p⁻-type epitaxial layer 12 and ann-type epitaxial layer 16. The p⁻-type epitaxial layer 12 is higher inspecific resistance than the semiconductor substrate 10. In thisembodiment, the epitaxial layer 12 is a semiconductor layer formed byepitaxially growing silicon which contains p-type impurities. Theepitaxial layer 16 is formed to provide a pn junction on the surface ofthe epitaxial layer 12. In this embodiment, the epitaxial layer 16 is asemiconductor layer formed by epitaxially growing silicon which containsn-type impurities.

In this embodiment, each of the semiconductor substrate 10, theepitaxial layer 12 and the epitaxial layer 16 consist of silicon.However, alternatively, these may be semiconductors that containgermanium, carbon or gallium.

The optical semiconductor device 100 maybe divided into two sections,i.e., a photo detector section 52 which receives light and generates aphotocurrent, and a signal-processing circuit section 60 which processesa signal based on the photocurrent generated by the photo detectorsection 52. In the epitaxial layer 16, a p⁺-type isolation 40 is formedbetween the photo detector section 52 and the signal-processing circuitsection 60 so as to isolate the photo detector section 52 from thesignal-processing circuit section 60.

The photo-detector section 52 includes an n⁺-type lead layer 42 and acathode electrode 28 connected to the lead layer 42 in the epitaxiallayer 16 in order to lead out the photocurrent generated by thephoto-detector section 52. An antireflection film 32 is formed on thesurface of the epitaxial layer 16 in a light-receiving region 52 a ofthe photo detector section 52 in order to prevent the reflection ofincident light incident on the light-receiving region 52 a. Thelight-receiving region 52 a is a region receiving the light in the photodetector section 52. The antireflection film 32 is directly formed onthe surface of the epitaxial layer 16 and no other film is present abovethe antireflection film 32. FIG. 1 shows a PIN photodiode as one exampleof the photo detector section 52. However, alternatively, a PNphotodiode may be used as another example of the photo detector section52.

The antireflection film 32 is formed out of a dielectric material suchas a silicon-nitride film or a silicon-oxide film. The silicon-nitridefilm can resist water content better than the silicon-oxide film and hasthe effect of being a passivation film. Therefore, the silicon-nitridefilm is particularly preferable as the antireflection film 32.

The signal-processing circuit section 60 includes various semiconductorelements to process the signal from the photo detector. FIG. 1 shows onebipolar transistor as one example of the semiconductor element. Theother examples of the semiconductor elements formed in thesignal-processing circuit section 60 involve a resistance, a capacitor,a MOSFET and the like.

Diffused layers necessary to form the signal-processing circuit section60 are formed in the epitaxial layer 16. In this embodiment, forexample, a base layer 44 b, an emitter layer 44 e and a collector layer44 c of the bipolar transistor are formed.

On the epitaxial layer 16, interlayer-insulating films, includinginsulating films 18 and 20, are formed so as to form a base electrode 29b, an emitter electrode 29 e and a collector electrode 29 c which havecontact with the base layer 44 b, the emitter layer 44 e and thecollector layer 44 c, respectively. The base electrode 29 b, the emitterelectrode 29 e and the collector electrode 29 c are each formed out ofmetal such as aluminum or copper. The insulating films 18 and 20 areeach formed out of a silicon oxide film. The insulating films 18 and 20are employed to insulate the electrodes 29 b, 29 e and 29 c from oneanother and to insulate the electrodes 29 b, 29 e and 29 c from anelectrode layer 22 to be described later.

Further, the electrode layer 22 is formed on the insulating film 20. Theelectrode layer 22 is a metal layer electrically connected to one of theelectrodes formed on the epitaxial layer 16 and also functions as alight-shielding film which shields the signal-processing circuit fromlight. It is thereby possible to prevent the semiconductor elementsformed in the signal-processing circuit section 60 from malfunctioning.The electrode layer 22 is one electrode layer of a multilayer wiringmade of metal. Passivation films 24 and 30 are further formed on theelectrode layer 22.

On the epitaxial layer 16 in the photo detector section 52 except forthe light-receiving region 52 a, the interlayer insulating filmsincluding the insulating films 18 and 20 are also formed to form acathode electrode 28. The cathode electrode 28 is made of metal such asaluminum or copper or the like. Further, the electrode layer 22 isformed on the insulating film 20, and the passivation films 24 and 30are formed on the electrode layer 22 as in the case of the epitaxiallayer 16 in the signal-processing circuit section 60.

The passivation film 30 and the antireflection film 32 serve aspassivation films which cover the outermost layers in thesignal-processing circuit section 60 and the photo detector section 52,respectively. In addition, the passivation film 30 is formed out of thesame material as that of the antireflection film 32. In this embodiment,the passivation film 30 and the antireflection film 32 are each formedout of a silicon nitride film. The passivation film 30 and theantireflection film 32 also cover a sidewall which consists of theinsulating films 18 and 20 and the passivation film 24 and which isprovided on a boundary between the signal-processing circuit section 60and the photo detector section 52. In other words, the passivation film30 and the antireflection film 32 are continuous to each other andformed out of the same single layer film.

The operation of the optical semiconductor device 100 in this embodimentas well as the advantages of the operation will now be described.

Light is incident on the light-receiving region 52 a of the photodetector section 52. This incident light reaches a depletion layerformed in a pn junction between the p⁻-type epitaxial layer 12 and then-type epitaxial layer 16 and generates a photocurrent. The photocurrentgenerated in the pn junction section is led out from the cathodeelectrode 28 through the lead layer 42 or an anode electrode (not shown)electrically connected to the epitaxial layer 12. Then, the photocurrentis processed by the signal-processing circuit section 60 as anelectrical signal.

The response rate of the photo detector is restricted by a CR timeconstant which is the product of the capacitance (C) and the resistancecomponent (R) of the photo detector, and is restricted by the runningtime of optically-excited carriers. In this embodiment, the photodetector section 52 is a PIN photodiode which has a low impurityconcentration in the epitaxial layer 12. The depletion layer, therefore,easily spreads in the epitaxial layer 12 at a low bias voltage. If thethickness of the epitaxial layer 12 is appropriately set, it is possibleto suppress the capacitance and resistance element of the photo detectorand to lower the CR time constant. Further, since the depletion layerspreads at a low bias voltage, it is possible to easily increase a fieldintensity in the depletion layer and to accelerate the running rate ofthe optically-excited carriers. As a result, the optical semiconductordevice 100 can deal with a high frequency signal.

In the conventional optical semiconductor device 200 shown in FIG. 8,the insulating films 18 and 20 and the passivation films 24 and 26formed in the photodiode 50 reflect most of the incident light. That is,the reflectance of the multilayer which consists of the insulating films18 and 20 and the passivation films 24 and 26 is high. In addition,because of the variation of the respective constituent films of themultilayer film, reflectance has great variation.

Generally, if a dielectric film which has a thickness d satisfying anexpression 1 is formed on a semiconductor material, the reflectance R ofthe surface of the dielectric film may possibly be a minimumreflectance. The minimum reflectance R is given by an expression 2.d=(λ/4n ₁)*(2m+1)  (Expression 1)R=(n ₀ n ₂ −n ₁ ²)²/(n ₀ n ₂ +n ₁ ²)²  (Expression 2)

In the expressions 1 and 2, symbol λ denotes the wavelength of incidentlight in vacuum. Symbol m denotes an integer not smaller than 0. Symboln₀ denotes the refractive index of a medium propagated by the lightbefore the light is incident on the dielectric film. This medium isoften nitrogen (N₂) and the refractive index n₀ is about 1. In thisembodiment, it is assumed that n₀ is 1. Symbol n₁ denotes the refractiveindex of the dielectric film. Symbol n₂ denotes the refractive index ofthe semiconductor material.

Conventionally, each of the insulating films 18 and 20 and thepassivation films 24 and 26 consists of the combination of a siliconoxide film and a silicon nitride film. The reflectance of the multilayerfilm which consists of the films 18, 20, 24 and 26 experimentally variesin a range of 10% to 40%. That is, the reflectance of the conventionalphotodiode 50 is high and the variation of the reflectance thereof islarge. In this case, the photo sensitivity of the conventional opticalsemiconductor device 200 varies from about 0.3 A/W (ampere/watt) toabout 0.43 A/W for light having a wavelength of 650 nm. Here, the photosensitivity is defined as a ratio of a photocurrent (A) to an incidentlight power (W).

On the other hand, in the optical semiconductor device 100 in thisembodiment according to the present invention, only the antireflectionfilm 32 is formed on the epitaxial layer 16 in the photo detectorsection 52. In this embodiment, the epitaxial layer 16 is a siliconlayer and has a refractive index (n₂) of about 3.44. The antireflectionfilm 32 is a silicon nitride film and has a refractive index (n₁) ofabout 2.05. The incident light has a wavelength λ of 650 nm. The mediumpropagated by the light before the light is incident on theantireflection film 32 is nitrogen (N₂). Therefore, n₀ is about 1. Inthis case, the thickness of the antireflection film 32 which satisfiesthe expression 1 is about 79.3 nm (m=0).

Using the expression 2, the reflectance R of the antireflection film 32at this moment is calculated as about 0.99%. As can be seen, thereflectance of the antireflection film 32 of the photo detector in thisembodiment is far lower than that of the conventional multilayer film.In addition, even if the thickness of the antireflection film 32 variesin a range of about 79.3 nm±10%, the reflectance R is always about 4% orless. Obviously, the variation of the reflectance is smaller than thatin the conventional art.

The antireflection film 32 which is formed out of a silicon nitride filmof 70 nm to 90 nm is experimentally formed on the epitaxial layer 16made of silicon. As a result, the photo sensitivity is in a range fromabout 0.49 A/W to about 0.50 A/W. Therefore, the photo sensitivity ofthe optical semiconductor device 100 in this embodiment improves beyondthat of the conventional optical semiconductor device. This is becausethe reflectance is lower than that of the conventional reflectance andthe quantity of light incident on the photo detector increases.

Under the same conditions as those in this embodiment, when quantumefficiency is 100% in theory, the photo sensitivity is 0.524 A/W. Thatis, the optical semiconductor device 100 in this embodiment can obtainthe photo sensitivity at quantum efficiency of about 95%. The quantumefficiency is the ratio of the number of charges which generate aphotocurrent to the light quantum of light incident on a photo detector.

Since no other film is present above the antireflection film 32 andsince the antireflection film 32 is a single layer film, it is possibleto control reflectance and photo sensitivity by controlling thethickness of the antireflection film 32. In this embodiment, therefore,it is possible to easily control the reflectance and the photosensitivity of the optical semiconductor device.

Since the antireflection film 32 is a single layer film, it is possibleto easily make the thickness of the antireflection film 32 thin.Normally, it is considered that the variation range of a film thicknessis about 10% of a desired film thickness. The variation of the filmthickness is made small because of the thin antireflection film 32. As aresult, the reflectance of the antireflection film 32 is stabilizedwithout causing variation thereof. In other words, the photo sensitivityof the optical semiconductor device 100 is stabilized.

The passivation film 30 and the antireflection film 32 are passivationfilms which cover the outermost layers in the signal-processing circuitsection 60 and the photo detector section 52, respectively. In addition,the passivation film 30 and the antireflection film 32 are formed out ofthe same single layer film (silicon nitride film in this embodiment) andcontinuous to each other. It is, therefore, possible to form thepassivation film 30 and the antireflection film 32 in the samemanufacturing step in the manufacturing process of the opticalsemiconductor device 100.

Furthermore, since the passivation film 30 and the antireflection film32 are continuous to each other, the passivation film 30 and theantireflection film 32 serve as effective passivation films forsemiconductor elements which are present below the passivation film 30and the antireflection film 32. That is, the antireflection film 32 hasnot only the antireflection function for preventing the reflection ofthe incident light, but also the function of a passivation film for thesemiconductor elements.

Next, a method for manufacturing the optical semiconductor device 100 inthis embodiment according to the present invention will be described.FIGS. 2 to 6 are partially enlarged cross-sectional views showing themethod for manufacturing the optical semiconductor device in thisembodiment in the order of manufacturing steps.

As shown in FIG. 2, first, the p⁻-type epitaxial layer 12 is formed onthe surface of the p-type semiconductor substrate 10. Next, the n-typeepitaxial layer 16 is formed on the epitaxial layer 12. The epitaxiallayers 12 and 16 can be formed by a vapor phase epitaxial growth method,a solid phase epitaxial growth method or the like. Prior to theformation of the epitaxial layer 16, the diffused layer 14 is formed inthe epitaxial layer 12. In this embodiment, the semiconductor substrate10 is a silicon substrate and the epitaxial layers 12 and 16 are bothformed out of silicon single crystal.

As shown in FIG. 3, such diffused layers as the element isolation layer40 for isolating the photo detector section 52 from thesignal-processing circuit section 60, the base layer 44 b, emitter layer44 e and collector layer 44 c for forming a bipolar transistor, and thelead layer 42 are next formed in the epitaxial layers 12 and 16. To formthese diffused layers, impurities are selectively injected into theepitaxial layers 12 and 16 and a heat treatment is then conducted. Theimpurities are arsenic (As) or phosphorus (P) as n-type impurities,boron (B) as p-type impurities and the like.

As shown in FIG. 4, the insulating film 18 is deposited by a CVD(Chemical Vapor Deposition) method or the like. The insulating film 18is patterned by photolithography and etching. Further, a metal layer isformed by a sputtering method and then patterned. As a result, theelectrodes 28, 29 b, 29 e and 29 c are connected to the diffused layers42, 44 b, 44 e and 44 c, respectively. In this embodiment, theinsulating film 18 is a silicon oxide film and the electrode 28 is madeof copper, aluminum or the like.

As shown in FIG. 5, the insulating film 20 is deposited to cover theseelectrodes 28, 29 b, 29 e and 29 c by the CVD method or the like. Inaddition, the electrode layer 22 is formed on the insulating film 20.The electrode layer 22 shields the signal-processing circuit section 60and the photo detector section 52 except for the light-receiving region52 a from incident light. The electrode layer 22 is often formed as onemetal wiring layer of a multilayer wiring. The electrode layer 22 isformed by sputtering the metal film and removing the metal film from thelight-receiving region 52 a by the photolithography and etching.Further, the passivation film 24 is deposited on the electrode layer 22and the insulating layer 20 by the CVD method or the like. In thisembodiment, the insulating film 20 and the passivation film 24 aresilicon oxide films and the electrode layer 22 is made of copper,aluminum or the like.

As shown in FIG. 6, the passivation film 24 and the insulating films 20and 18 are removed from the light-receiving region 52 a by thephotolithography and etching. As a result, the surface of the epitaxiallayer 16 in the light-receiving region 52 a is exposed.

Alternatively, the epitaxial layer 16 in the light-receiving region 52 amay be exposed by forming a plug in the light-receiving region 52 a inadvance, selectively depositing the insulating films 18 and 20, theelectrode layer 22 and the passivation film 24 on the epitaxial layer 16and then removing the plug.

As shown in FIG. 7, the passivation film 30 and the antireflection film32 are deposited so as to cover the signal-processing circuit section 60and the photo detector section 52. The passivation film 30 covers thepassivation film 24 in the signal-processing circuit section 60 and inthe photo detector section 52 except for the light-receiving region 52a. The antireflection film 32 covers the surface of the exposedepitaxial layer 16 in the light-receiving region 52 a. In thisembodiment, the passivation film 30 and the antireflection film 32 areeach formed out of a silicon nitride film. The passivation film 30 andthe antireflection film 32 may be formed simultaneously in the samemanufacturing step. The passivation film 30 and the antireflection film32 may be deposited by, for example, an LP-CVD (Low-Pressure ChemicalVapor Deposition) method.

According to the method for manufacturing the optical semiconductordevice in this embodiment, the passivation film 30 and theantireflection film 32 are formed simultaneously in the samemanufacturing step. This makes it possible to easily manufacture theoptical semiconductor device according to the present invention. Morespecifically, only a photolithographic step and an etching step areadded so as to remove the films 24, 20 and 18 from the light-receivingregion 52 a. Therefore, according to the method for manufacturing theoptical semiconductor device in this embodiment, it is possible toeasily manufacture the optical semiconductor device according to thepresent invention and to hold down the manufacturing cost.

Further, according to the method for manufacturing the opticalsemiconductor device in this embodiment, the passivation film 30 and theantireflection film 32 are formed in the final step of the opticalsemiconductor device manufacturing steps. Due to this, no other film butthe antireflection film 32 is formed on the antireflection film 32. Inother words, the film formed on the epitaxial layer 16 is a single layerfilm consisting only of the antireflection film 32. It is therebypossible to determine the reflectance of the optical semiconductordevice by the thickness of the antireflection film 32. In addition, bymaking only the antireflection film 32 thin, it is possible to decreasethe variation of the reflectance.

Furthermore, since the passivation film 30 and the antireflection film32 are formed simultaneously in the same manufacturing step, the films30 and 32 also cover a sidewall which consists of the films 18, 20 and24 provided on a boundary between the signal-processing circuit section60 and the photo detector section 52. Therefore, the films 30 and 32 areformed to be continuous to each other. It is thereby possible to enablethe passivation film 30 and the antireflection film 32 to function moreeffectively as passivation films.

Meanwhile, Japanese Patent Application Publication No. 4-271173discloses an optical semiconductor device having a dielectric thin filmand an antireflection film which have common properties and thicknessand which are manufactured in a common manufacturing step. Thedielectric thin film is used between the electrodes of the capacitor ofa peripheral circuit element. The antireflection film is used in a photodetector.

In the optical semiconductor device disclosed in Publication No.4-271173, the thickness of the antireflection film is a factor whichdetermines the capacitance of the capacitor. Therefore, the thickness ofthe antireflection film is limited by the capacitance of the capacitor.If the thickness of the antireflection film is set at an optimumthickness in accordance with the wavelength of incident light, the areasof the electrodes of the capacitor have to be changed so as to obtain adesired capacitance. For example, it is sometimes necessary to set theelectrode areas of the capacitor to be larger than those in an ordinarycase in order to obtain the desired capacitance while maintaining thethickness d of the antireflection film which satisfies the expression 1.

In the optical semiconductor device disclosed in Publication No.4-271173, the antireflection film of the photo detector is formed whenthe dielectric thin film used between the electrodes of the capacitor isformed. Due to this, such films as passivation films are formed on theantireflection film of the photo detector. As a result, in order tocontrol the reflectance in the photo detector, it is disadvantageouslynecessary to control not only the thickness of the antireflection filmbut also those of the passivation films on the antireflection film.

In the optical semiconductor device 100 in the embodiment according tothe present invention, by contrast, only the single layer filmconsisting of the antireflection film 32 exists on the epitaxial layer16 in the photo detector section. Therefore, it suffices to control onlythe thickness of the antireflection film so as to control thereflectance of the photo detector. In addition, in the opticalsemiconductor device in the embodiment according to the presentinvention, the antireflection film 32 also functions as a passivationfilm, irrespectively of the capacitance of the capacitor. As a result,even if the antireflection film 32 maintains the thickness d whichsatisfies the expression 1, no problem occurs to the signal-processingcircuit section 60 as the capacitor and the like.

As stated so far, according to the optical semiconductor device and themethod for manufacturing the optical semiconductor device in thisembodiment, even if the photo detector and the signal-processing circuitelements are formed on the same semiconductor substrate, it is possibleto set the photo sensitivity of the optical semiconductor device higherthan that of the conventional optical semiconductor device and to makethe variation of the photo sensitivity smaller than that of theconventional optical semiconductor device.

According to the optical semiconductor device and the method formanufacturing the optical semiconductor device in this embodiment, evenif the photo detector and the circuit elements are formed on the samesemiconductor device, it is possible to control the photo sensitivityrelatively easily without influencing the circuit elements.

According to the optical semiconductor device and the method formanufacturing the optical semiconductor device in this embodiment, it ispossible to form the photo detector and the signal-processing circuitelements which have relatively high photo sensitivity and smaller rangeof the variation of the photo sensitivity, on the same semiconductorsubstrate.

1. A method for manufacturing an optical semiconductor device having aphoto detector section to receive light and generate a photocurrent anda circuit element section to process a signal based on the photocurrentfrom said photo detector section on a same semiconductor substrate, themethod comprising: forming a first semiconductor layer of a firstconductivity type on a surface of the semiconductor substrate; forming asecond semiconductor layer of a second conductivity type on a surface ofsaid first semiconductor layer; forming selectively diffused layers insaid second semiconductor layer in said photo detector section and saidcircuit element section; depositing a first insulating film on saidsecond semiconductor layer; exposing said second semiconductor layer ina light-receiving region which receives the light in said photo detectorsection; and forming an antireflection film on said second semiconductorlayer in said light-receiving region, and forming a passivation filmabove said first insulating film, said antireflection film preventing areflection of an incident light, said passivation film being made of asame material as a material of said antireflection film and coveringsaid circuit element section, and said antireflection film beingcontinuous with said passivation film.
 2. A method for manufacturing anoptical semiconductor device according to claim 1, wherein saidantireflection film and said passivation film are formed simultaneouslyin a same manufacturing step.
 3. A method for manufacturing an opticalsemiconductor device according to claim 2, wherein said antireflectionfilm and said passivation film are formed in the final step of theoptical semiconductor manufacturing step.
 4. A method for manufacturingan optical semiconductor device according to claim 3, wherein each ofsaid antireflection film and said passivation film is a silicon nitridefilm.
 5. A method for manufacturing an optical semiconductor deviceaccording to claim 4, wherein said antireflection film and saidpassivation film are deposited by a CVD method.
 6. A method formanufacturing an optical semiconductor device according to claim 1,wherein each of said antireflection film and said passivation film is asilicon nitride film.
 7. A method for manufacturing an opticalsemiconductor device having a photo detector section to receive lightand generate a photocurrent and a circuit element section to process asignal based on the photocurrent from said photo detector section on asame semiconductor substrate, the method comprising: forming a firstsemiconductor layer of a first conductivity type on a surface of thesemiconductor substrate; forming a second semiconductor layer of asecond conductivity type on a surface of said first semiconductor layer;forming selectively diffused layers in said second semiconductor layerin said photo detector section and said circuit element section;depositing a first insulating film on said second semiconductor layer;patterning said first insulating film, and forming electrodes connectedto said diffused layers; depositing a second insulating film coveringsaid electrodes; forming at least one electrode layer on said secondinsulating film, said electrode layer being electrically connected toone of said electrodes; exposing said second semiconductor layer in alight-receiving region which receives the light in said photo detectorsection by etching said first insulating film and said second insulatingfilm which are present in said light-receiving region; and forming anantireflection film on said second semiconductor layer in saidlight-receiving region, and forming a passivation film above said firstinsulating film, said antireflection film preventing a reflection of anincident light, said passivation film being made of a same material as amaterial of said antireflection film and covering said circuit elementsection, and said antireflection film being continuous with saidpassivation film.
 8. A method for manufacturing an optical semiconductordevice according to claim 7, wherein said antireflection film and saidpassivation film are formed simultaneously in a same manufacturing step.9. A method for manufacturing an optical semiconductor device accordingto claim 8, wherein said antireflection film and said passivation filmare formed in the final step of the optical semiconductor manufacturingstep.
 10. A method for manufacturing an optical semiconductor deviceaccording to claim 9, wherein each of said antireflection film and saidpassivation film is a silicon nitride film.
 11. A method formanufacturing an optical semiconductor device according to claim 10,wherein said antireflection film and said passivation film are depositedby a CVD method.
 12. A method for manufacturing an optical semiconductordevice according to claim 7, wherein each of said antireflection filmand said passivation film is a silicon nitride film.
 13. A method formanufacturing an optical semiconductor device according to claim 7,wherein said antireflection film and said passivation film are depositedby a CVD method.
 14. A method for manufacturing an optical semiconductordevice having a photo detector section to receive light and generate aphotocurrent and a circuit element section to process a signal based onthe photocurrent from said photo detector section on a samesemiconductor substrate, the method comprising: forming a firstsemiconductor layer of a first conductivity type on a surface of thesemiconductor substrate; forming a second semiconductor layer of asecond conductivity type on a surface of said first semiconductor layer;forming selectively diffused layers in said second semiconductor layerin said photo detector section and said circuit element section;depositing a first insulating film on said second semiconductor layer;exposing said second semiconductor layer in a light-receiving regionwhich receives the light in said photo detector section; and forming asingle layer film on said circuit element section and said photodetector section, said single layer film acting as an antireflectionfilm preventing a reflection of an incident light in saidlight-receiving region and acting as a passivation film protecting thecircuit element in said circuit element section.
 15. A method formanufacturing an optical semiconductor device according to claim 14,wherein said antireflection film and said passivation film are formedsimultaneously in a same manufacturing step.
 16. A method formanufacturing an optical semiconductor device according to claim 15,wherein said antireflection film and said passivation film are formed inthe final step of the optical semiconductor manufacturing step.
 17. Amethod for manufacturing an optical semiconductor device according toclaim 16, wherein each of said antireflection film and said passivationfilm is a silicon nitride film.
 18. A method for manufacturing anoptical semiconductor device according to claim 17, wherein saidantireflection film and said passivation film are deposited by a CVDmethod.
 19. A method for manufacturing an optical semiconductor deviceaccording to claim 14, wherein each of said antireflection film and saidpassivation film is a silicon nitride film.
 20. A method formanufacturing an optical semiconductor device according to claim 14,wherein said antireflection film and said passivation film are depositedby a CVD method.